Transponders are typical systems that use inductive coupling for data transfer and also for energy transfer. The inductive interface includes an antenna coil and a capacitor forming a resonant circuit, and a rectifier with a storage capacitor. Since a transponder is supplied from inductively transmitted energy, it may be battery-less. An example of a battery-less transponder is a vehicle immobilizer. A vehicle immobilizer is a portable device that may include a physical key fitting into a lock, in addition to a transponder. While the basic function of such a device is that of a reliable key, other features may be added. For example, remote control functionality is among the features that respond to the ongoing demands for comfort. Remote control generally involves a RF transceiver, which needs to be powered from a battery. In spite of their increasing complexity, these systems are mass products and it is essential to keep the system costs as low as possible. A considerable reduction of the total system cost is achieved by combining plural functions in a single circuit and by sharing resources such as a controller, encryption circuits or memory, allowing to reduce the chip size. The shared circuit parts must be supplied regardless whether a supply is available from the inductive interface or from a battery. Since each power supply can have a voltage between zero and a maximum voltage up to e.g. 8 volt, an intelligent power management is required. The power management should make the best use of energy received over the inductive interface to preserve battery lifetime. In addition, energy received over the inductive interface in excess of that required for the basic functionality, should be made available to charge the battery (if a rechargeable battery is used). Also, conflicts between the power supplies must be avoided.